(1) Field of the Invention
The present invention relates to a system linking apparatus for generated electric power which is constructed to connect an electric power system to an output side of a power generator through a linking inverter. More particularly, the present invention relates to the system linking apparatus for generated electric power which does not suffer from increase of a current capacity of a converting device resulting from an inferior power factor caused by leakage inductance of a power generator.
(2) Description of the Related Art
Recently, aero generation that utilizes inexhaustible and regenerative energy, that is, wind power is rapidly prevailing worldwide as an environment-friendly power generation system. In such a wind-power generation system, today, attention is focused to a variable-speed technique that operates to greatly reduce an output variation of a power generator.
In a case that a power generator is driven by a windmill to be rotated by wind energy and electric power generated by the wind-power generator is regenerated in a commercial electric power system, it is necessary to reduce the generation cost and improve the reliability of the generation and further to obtain effective energy based on efficient regeneration of electric power. This kind of problem about the system linkage is disclosed in the Official Gazette of Japanese Patent No. 2897208. Further, the reduction of output variation, the reduction of rush current, the lowering of noise, and the amelioration of a maintenance capability are described in Isamu NAGATA, et al: “Development of Gearless Variable-speed Wind Turbine” (Technical Report of Mitsubishi Heavy Industry, Ltd., Vol 38, No. 2, pp. 100-103, March 2001).
FIG. 4 is a circuit diagram showing a generation system which connects an electric power system to an output of the conventional power generator through a linking inverter. This generation system is constructed to have a generator 1 coupled with a rotation shaft of a propeller 100, a PWM rectifier 9 connected with the output side of the power generator 1 through a filter 8, a capacitor 4 and a PWM inverter 5 both of which are connected on the dc output side of the PWM rectifier 9, and a filter 6 connected between an output of the PWM inverter 5 and an electric power system 7. Herein, each of the PWM rectifier 9 and the PWM inverter 5 is composed of a three-phase bridge circuit having six switches, each of which is made up of an IGBT and a diode connected in inverse-parallel thereto.
FIG. 5 shows an arrangement of a converter included in the power generation system shown in FIG. 4 and the operation waveform of the converter. FIG. 6 is a Feather view showing relation between a voltage vector and a current vector in the power generation system.
FIG. 5B shows the current and voltage waveforms appearing in the case of controlling an output voltage Vu of the PWM rectifier 9 to which an IGBT gate signal of the three-phase bridge circuit is supplied so that a power factor 1 may be derived from an armature voltage Vuint and an output current Iu of the power generator 1. The power generator 1 shown in FIG. 5A changes its terminal voltage according to a magnitude and a phase of current outputted from the power generator 1. Hence, in order to flow such current that the voltage Vuint of each armature winding and the output current of the power generator 1 are mixed into a power factor 1, it is necessary to output an output voltage Vu whose amplitude is larger than the voltage Vuint of the armature into the PWM inverter 5 by delaying the phase of the output voltage Vu.
In FIG. 6, the voltage Vx is a voltage generated by a leakage reactance X in the armature winding of the power generator 1. The magnitude of this voltage is determined by a product of the leakage reactance X and the output current Lu of the power generator.
Letting a control phase angle as θ, the output power P0 of the power generator 1 is represented by the following expressions (1) and (2);P0=3·Vu·Iu·cos θ  (1)P0=3·Vuint·Iu  (2)And,Cos θ=1/√{square root over (1+x2)}  (3)wherein x denotes a percent impedance of the leakage reactance X.
Hence, since the capacity of the PWM rectifier 9 is 3·Vu·Iu, the electric power outputted by the power generator 1 is required to be √{square root over (1+x2)} times as large as that provided in the case that the power factor is 1. Hence, the capacity of the converter of the PWM inverter 5 is required to be √{square root over (1+x2)} times as large as that provided in the case that the power factor is 1.
The PWM inverter 5 and the filter 6 control the output voltage in the phase synchronized with the electric power system 7, so that the electric power condensed in a capacitor 4 may be outputted to the electric power system 7. At a time, the PWM inverter 5 serves as a linking converter for controlling the phase and the voltage value of the output voltage. As such, the PWM inverter 5 controls a magnitude of an electric power to be outputted to the electric power system 7.
The conventional system linking apparatus requires the PWM rectifier 9 to have (1+x2)1/2 times as large a capacity. Hence, the PWM rectifier 9 is made larger in size. Further, since the PWM rectifier 9 needs to output (1+x2)1/2 times as large an output voltage Vu, the PWM rectifier 9 needs to be composed of a highly endurable semiconductor device. In general, therefore, a highly endurable semiconductor device brings about a larger conduction loss and switching loss than a low endurable semiconductor. These power losses bring about a difficulty in effectively obtaining energy.
Further, since the PWM rectifier 9 attenuates harmonic components of a carrier frequency, a filter 8 is required to be connected with the PWM rectifier 9. For reducing the filter 8 in size, in general, a high-frequency filter of several kHz or more is used. Hence, the PWM rectifier 9 disadvantageously brings about a larger switching loss and makes the system linking apparatus larger in size and more costly.